Envelope for X-ray generator



Feb. 20, 1962 z. J. ATLEE 3,022,435

ENVELOPE FOR X-RAY GENERATOR Filed Dec. 22. 1958 FIG. 2

INVENTOR- ZED J. ATLEE BY A T TORNEVJ Fl nn; a

United States Patent 3,022,435 ENVELOPE FOR X-RAY GENERATOR Zed J. Atlee, Chicago, Ill., assignor to Dunlee Corporation, Chicago, 111., a corporation of Illinois Filed Dec. 22, 1958, Ser. No. 782,993 6 Claims. (Cl. 313-59) The present invention relates to electron discharge devices and more particularly to an envelope construction for electron discharge devices such as X-ray generators.

Electron discharge devices such as X-ray generators, with which this invention has particular concern, comprise an electron generating cathode and an electron receiving anode mounted in spaced relation within an envelope formed of glass or similar material. Electrical potential is applied between the electrodes to accelerate electrons from the cathode to the anode. Some of the electrons unavoidably impinge upon the inner wall of the envelope, and, if not drawn 01f, tend to collect and establish charges which may cause a puncture of the envelope or produce potential gradients adversely affecting the operation of the device.

A particular problem with X-ray generators has arisen in connection with fluorescence developing in the glass envelope during operation of the generator. The fluorescence may cause a charge to develop on the envelope wall which may deflect the focal spot and fluorescence may also cause the envelope to overheat. To obviate the problems in connection with fluorescence in X-ray tubes made in accordance with the prior art, for example, such as those made in accordance with the teachings of my prior patent, Atlee et al. 2,640,167, it frequently has been necessary to provide the interior surfaces of the tube envelope with a conductive material such as aquadag. Such coatings are not required in X-ray tubes made in accordance with the present invention.

It is a principal object of the present invention to provide an electron discharge device such as an X-ray generator having an envelope of glass with electrical conductivity characteristics such that electrostatic charges arising in the envelope are substantially immediately drained off or dissipated so that excessive charges do not develop.

More particularly, it is an object of the invention to provide an X-ray generator capable of operation at higher voltages than heretofore possible.

It is another important object of the invention to provide an electron discharge device such as an X-ray generator having an envelope of glass characterized by its lack of fluorescence during operation of the device.

Another object is to provide an electron discharge device having a unitary envelope of glass of a relatively low softening temperature, thus facilitating manufacture and assembly of the device.

A further object of the invention is to provide an electron discharge device with an envelope of glass having substantially the same coefficient expansion as iron-nickel alloys such as Kovar.

Another object is to provide an X-ray generator having a unitary glass envelope of high X-ray transparency.

Other objects and advantages of the invention will become apparent hereinafter.

In accordance with the illustrated and herein described embodiment of the invention, electron discharge devices are provided with an envelope of glass having a resistivity of about 10 (5.075) ohms/cm. at 350 degrees C. and 10 (6.430) ohms/emf at 250 degrees C.; a softening temperature of 664 degrees C.; an annealing temperature of 505 degrees C.; a strain point of 476 degrees C.; a thermal coefficient of expansion of 4.75 10 cm./cm./ C.; and a very high transparency to X-rays.

For a more detailed description of the invention, reference is made to the accompanying drawings and following specification.

Referring to the drawings:

FIG. 1 is a side elevational view of an X-ray generator made in accordance with the present invention; and

FIG. 2 is a longitudinal sectional view of the generator.

The invention is illustrated in connection with an X-ray generator including an anode 10 and a cathode 12 mounted in facing relation in the opposite ends of an envelope 14 of glass, the characteristics and composition of which will be described in greater detail hereinafter.

The anode 10 may comprise a copper body 16 and is provided at its end facing the cathode with an X-ray generating target 18, which may be of Wolfram or other suitable material, and at its other end with a stem 20 which extends outwardly of the envelope to provide means for cooling the anode. The anode 10 is shown as supported on a sleeve 22, preferably of a high nickel-iron alloy such as Kovar, the sleeve being joined to a re-entrant end of the envelope by a glass-to-metal seal indicated at 24.

The cathode 12 is illustrated as comprising an elongate sleeve 30, one end of which fits telescopically over the end of a re-entrant, cylindrical glass stem portion 32 of the envelope 14 and is secured thereto by wire wrappings 34. Mounted in the opposite end of the sleeve 30 is a cathode head block 35 formed with an electron focusing cup in which is mounted a filament 36 connected to electrical leads 38 which extend outwardlyof the envelope through a pinch stem 40. As is known, electrons generated at the filament 36 are directed in a beam 42 upon the anode target 18, resulting in the emission of X-rays indicated at 44.

As was indicated previously, during the operation of an X-ray generator some of the electrons emitted by the filament will stray from the beam 42 and Will impinge .upon the envelope 14. If such electrons are permitted to collect, charges may develop on the inner Wall of the envelope, resulting in a potential drop through the envelope sufficient to cause puncture of the same. I have found that if the envelope 14 is formed of glass having an electrical resistivity of between about 10 and 1 0 ohms/cm? at 350 degrees C., the charges arising from the impingement of electrons upon the envelope will be drained oft at such a rate as to substantially eliminate puncturing of the envelope. Preferably the glass should have a coefiicient of about 4.7x 10- crn./cm./ C., so as to facilitate its union to Kovar iron-nickel alloy and should have a relatively low softening point to facilitate working of the glass to shape the envelope and assemble the generator.

A specific glass having the desired characteristics is an alumino-borosilicate glass comprising the following materials:

This glass has a strain point of 476 degrees C., and an annealing point of 505 degrees C., and a softening point of 664 degrees C. The thermal coefiicient of expansion of this glass is 4.75 1O- cm./cm./ C. and its resistivity at 350 degrees is 10 ohms/cm. and at 250 degrees C. is 10 ohms/cmfi.

In addition this glass has the very desirable quality in that it does not exhibit fluorescence during operation of an X-ray generator. As mentioned previously, the occurrence of fluorescence causes undesirable heating of the envelope and the development of wall charges which may distort or deflect the electron beam causing, in turn, distortion of the pattern of generated Xrays. Because of the lack of fluorescence, it is not necessary to provide a conductive coating on the surface of an envelope made of the glass and which coatings were, as mentioned earlier, frequently necessary with prior art envelopes.

The glass of the example also has a very high transparency to X-rays, an obviously desirable feature. Compared to aluminum, for example, glass of the specific example of 0.100" thickness will pass about 13.5 roentgens per minute whereas aluminum of the same thickness Will pass only about 6.0 roentgens per minute of X-rays from a source of like intensity.

One of the very desirable advantages of an X-ray generator provided with an envelope in accordance with the invention is that the generator may safely be operated at higher voltages than would be possible with an envelope made in accordance with the prior art.

The thermal coefficient of expansion of the glass is substantially the same as that of Kovar iron-nickel alloy and the glass may be readily sealed to Kovar which is used to make the glass-to-metal seal at the anode end of the generator. conventionally, the filament leads such as leads 38 of an X-ray generator are formed of Wolfram. However, it was found that a satisfactory seal to Wolfram leads could not be made with the glass of the example. I have discovered, however, that the glass will wet molybdenum and that a satisfactory seal may be made if the leads 38 are formed of molybdenum wire.

It will be noted that the softening temperature 664 degrees C. of the glass is relatively low, which facilitates manipulation of the glass in the construction of the envelope and assembly of the generator.

Having illustrated and described a preferred embodiment of the invention, it should be'apparent to those skilled in the art that the invention permits of modification in arrangement and detail. I claim as my invention all such modifications as come within the true spirit and scope of the appended claims.

This application is a continuation-in-part of my prior application Serial No. 587,883, filed May 28, 1956, now abandoned.

I claim: 1. An X-ray generator comprising an anode and a cathode, and an evacuated envelope enclosing said anode and cathode, said envelope being formed of glass comprising about the following constituents:

lack of fluorescence during operation of the generator, and its resistance to electron puncture.

2. An X-ray generator comprising an anode and a cathode, and an evacuated envelope enclosing said anode and cathode, said envelope being formed of glass having high X-ray transmissibility and an electrical resistivity of about ro ohms/cm. at 350 degrees C. and about 10 ohms/cm. at 250 degrees C. whereby electrical charges accumulating on the surface of the envelope are rapidly drained away so that areas of high electrical stress do not develop and characterized by its lack of fluorescence during operation of the generator.

3. An X-ray generator as set forth in claim 2 wherein said glass has a softening temperature of about 664 degrees C.

4. An X-ray generator as set forth in claim 2 wherein said glass has an annealing temperature of about 505 degrees C.

5. An X-ray generator as set forth in claim 2 wherein said glass has a strain temperature of about 476 degrees C.

6. An X-ray generator as set forth in claim 2 wherein said glass has a coefficient of expansion of about 4.75 10 crn./cm./ C.

References Cited in the file of this patent UNITED STATES PATENTS 2,582,852 Shoemaker Jan. 15, 1952 2,597,562 Blodgett May 20, 1952 2,640,167 Atlee et al. May 26, 1953 2,743,553 Armistead May 1, 1956 2,833,953 Rogers May 6, 1958 

1. AN X-RAY GENERATOR COMPRISING AN ANODE AND A CATHADE AND AN EVACUATED ENVELOPE ENCLOSING SAID ANODE AND CATHODE, SAID ENVELOPE BEING FORMED OF GLASS COMPRISING ABOUT THE FOLLOWING CONSTITUENTS; 